In the mammalian central nervous system (CNS), the transmission of nerve impulses is controlled by the interaction between a neurotransmitter substance released by the "sending" neuron and a surface receptor on the "receiving" neuron to which the neurotransmitter binds causing excitation thereof. There are a number of neurotransmitters in the CNS, each of which target specific receiving neurons. For example, glutamate, dopamine and serotonin neurotransmitters each target a different family of receptors. Glutamate, which is referred to as an excitatory amino acid (EAA), interacts with receptors variously referred to as glutamate or EAA receptors, while dopamine and serotonin interact specifically with dopamine and serotonin receptors, respectively.
Within each receptor family, the receptors are classified by their ligand-binding or functional characteristics. For example, some EAA receptors are classified according to their differential binding to the agonists, NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate), and kainate (2-carboxy-4-(1-methylethenyl)-3-pyrrolidineacetate). Thus, NMDA receptors bind glutamate and bind NMDA-with greater affinity than kainate or AMPA, while AMPA and kainate receptors bind glutamate, and bind AMPA and kainate, respectively, with greater affinity than other agonists.
In contrast to dopamine and serotonin receptors, some EAA receptors are functional in an electrophysiological sense as determined by established electrophysiological assays such as that described by Hollman et al. in Nature 342: 643, 1989, or by any other assay appropriate for detecting conductance across a cell membrane. In essence, EAA receptors form ligand-gated ion channels. Thus, in response to binding an appropriate ligand, e.g., glutamate, AMPA, kainate or NMDA, an EAA receptor ion channel will "open" or become more permeable to allow the influx of cation that is required for normal synaptic transmission. In the absence of ligand binding, the ion channels remain "closed" or less permeable to cation, preventing the inward flow of cation required for synaptic transmission.
At least six AMPA-type rodent receptors have been cloned, and named GluR-1 to 6. Expression studies suggest that GluR-2 is the dominant subunit in determining functional properties associated with Ca.sup.2+ permeability in this rodent receptor family. Mutation studies have shown that this permeability is determined by a single amino acid, arginine (R), in the putative channel-forming transmembrane II (TMII) of rat GluR-2; a glutamine (Q) residue is present in the other AMPA receptors. It was subsequently revealed that the R form of the GluR-2 receptor is generated from the same gene as the Q form by an RNA editing process, indicating that, in rat brain, the occurrence of this "editing" process determines cation flow in GluR-2 channels (Sommer et al, 1991, Cell, 67:11). Reports to date have found almost 100% efficiency of the editing process for rodent GluR-2 with low level expression of unaltered Q forms in the developing central nervous system (Sommer et al, supra; and see Burnashev et al, 1992, Neuron, 8:189). Most recently, the AMPA-type rat receptors GluR-5 and GluR-6 have also been shown to undergo RNA editing (Sommer et al, supra; Burnashev et al, supra; and see Kohler et al, 1993, Neuron, 10:491).
RNA editing is a relatively rare phenonmenon, but occurs in various organisms and may involve a number of different mechanisms. The editing of the rodent AMPA receptor, GluR-2, has been demonstrated to require a base paired intron/exon structure.
A nuclear adenosine deaminase specific for double-stranded DNA is postulated to be involved in the base conversion, although direct evidence of the mechanism and any regulation of the process remain to be investigated.
Several human glutamate receptors have been cloned, including those of the AMPA-type such as hGluR1 (Puckett et al, 1991, Proc. Natl. Acad. Sci., 88:7557), hGluR-2, hGluR-3 (Biochem. Biophys. Acta, 1994, 1219:563) and those of the kainate-type, such as humEAA1 (EP 529,994); humEAA2 (EP 529,995); humEAA3 (EP 617,123) and humEAA4 (EP 578,409). The human glutamate receptors are of great medical importance because of their postulated role in the mediation of learning and memory acquisition. In addition, excitatory amino acids can be highly toxic to neurons and dysfunction of this neurotransmitter system has been implicated in several neurological disorders such as Alzheimer's disease, Huntington's chorea, epilepsy, Parkinson's disease, amyotrophic lateral sclerosis, AIDS encephalopathy and dementia complex. To date, the RNA editing phenomenon, an important determinate of the functional properties of CNS receptors and particularly glutamate receptors, has not been observed in humans.